Conventionally, an image processing technique of extracting the contour lines of a raster-input binary image, and storing the binary image as contour vector data expressed by the contour lines has been proposed.
Furthermore, for a pseudo halftone image including many isolated points, a technique of avoiding an increase in memory capacity by separately storing position information of each isolated point on an image in place of storing the contour of each isolated point as vector data one by one has also been proposed as a technique to be added to the above-mentioned one.
In the above-mentioned technique which separately handles isolated points in a pseudo halftone image, when an input image is to be output in an enlarged or reduced scale, each isolated point is finally output by merely increasing the number of pixels to an integer multiple approximate to a desired variable magnification factor. For this reason, if the variable magnification factor is not an integer multiple like .times.2.5, the density of a pseudo halftone portion of the output image becomes different from that of the input image.
An image processing technique of extracting the contour lines of a raster-input binary image, and storing the binary image as contour vector data expressed by the contour lines has been conventionally proposed. In association with a technique of this type, the present applicant has already filed Japanese Patent Laid-Open No. 4-157578. Furthermore, a technique of converting a binary image stored as contour vector data into a raster format as a standard data format has also been conventionally proposed. For example, the present applicant filed Japanese Patent Laid-Open Nos. 5-40831, 5-20466, 5-20467, and 5-20468. Also, as a technique of obtaining a satisfactory enlarged/reduced image using contour vector data of a binary image, Japanese Patent Application No. 3-345062 has been proposed.
With the above-mentioned technique of converting a binary image into contour vector data, geometric modification processing such as enlargement/reduction, rotation, and the like can be easily attained. However, a binary image includes components constituted by characters and line images, and pseudo halftone components which express a density pattern, and it is often preferable to adaptively perform processing operations of different algorithms for these components depending on the modification processing contents.
In particular, when the conversion technique is applied to variable-magnification processing like in the technique proposed by Japanese Patent Application No. 3-345062, a satisfactory enlarged/reduced image can be obtained based on an image constituted by character/line image components. However, when the technique of this proposal is directly applied to an image constituted by pseudo halftone components, the image quality suffers. For this reason, adaptive variable-magnification processing must be performed for these two different types of components.